A new approach is needed to tackle the climate crisis, in which the long overlooked supply-side of fossil fuels takes centre stage. A crucial aspect of this is the need for international agreements and law to effectively and fairly leave large swathes of remaining fossil fuels in the ground. Towards that end, we make the case for a Fossil Fuel Non-Proliferation Treaty (FF-NPT) to deal with emissions at source. Having explained the need for such a treaty, we propose overall aims, and both a process and principles for the sequencing of efforts across fuel types and regions based on equity and justice. We discuss the form an FF-NPT could take, as well as some of the key challenges it would have to overcome. We suggest strategies for overcoming key challenges in relation to reserves in developing countries, questions of the just transition, and incentives for countries to sign up to such a treaty.